# Why do some SMPS power supplies require an input voltage select switch?

I have 2 SMPS power supplies near me. One is an ATX power supply, the other is for a 3d printer. They both are rated for 110-240v in, but the ATX supply can handle the full range automatically, while the other supply has an input voltage select switch

One quirk I noticed with the supply with the switch, if I flip that switch to 230v, with my 115v mains, the output is half the rated voltage. I assume that is not a good idea to do that.

• What does that switch do?
• What is so different about these power supplies that one needs a switch and one does not?
• What are the risks of having the switch in the wrong position?

Most likely the switch controls how the mains input is rectified into the input bulk capacitors. This was common on older ATX power supplies as well, to use single design that can be used globally. Today the PFC stage at the input can handle larger input range automatically.

In the supply, there might be two 200V bulk capacitors for storing rectified mains. In the 230V position, the bridge rectifier is connected as a full bridge to the two series connected capacitors to effectively use them as a single 400V capacitor. Stored voltage is about 325V peak, or about 162V per capacitor. In the 115V position, the bridge rectifier is connected as a half bridge to a single capacitor, where positive mains cycle rectifies the 162V peak into one capacitor, and negative mains cycle rectifies the 162V peak into other capacitor. The total voltage over both capacitors would then be again 325V.

So if you use the switch in 230V position and feed in 115V, the input voltage is half what is needed and due to undervoltage it may not work at all or it might be so simple design that it just blindly tries to work but output halved as well. As this is not intended operation it may damage the parts.

Using it the other way, switch in 115V position but feeding in 230V will be much more dangerous and spectacular. The fuse might blow immediately, and if it does not, there will be a lot of overvoltage at the capacitors so they can heat up and vent up electrolytes as vapour after a while (seen this) or the capacitors may just explode outright.

That switch could do anything. More often than not, especially on Chinesium power supplies, having a switch like that indicates a lack of a PFC stage.

1) That switch most likely switches which taps on their transformer are used. Say their supply needs 60V on the secondary, for 120 you need a 2:1 ratio, for 240 you need a 4:1 ratio. They may have 2 2:1 primaries that are either wired in series or parallel based on that switch for both voltage ranges. This may be why your output voltage is half of what it should be. Different cheapo supplies use different methods. 120 uses twice the current as 240 for the same amount of power so things need to be sized/done differently.

2) Like I said earlier, your ATX supply most likely has a PFC stage which takes the AC input and boosts it to ~400VDC before the DC/DC converters turn that back into 5, 12, etc. Because the boost can work on anything less than 400Vpk, the AC input can be pretty much anything (the "universal range" is 90-264VAC).

3) The risks? Fire, death, bodily injury, you know, the usual things. If everything in the supply is sized for the 120V currents, then it probably just won't work well, but copper is expensive and corners can be cut. If you see an agency label on it then they test all these things and you'll be fine either way.

Figure 1. The AC input section of STEVAL-ISA018V1: 150W, 24V @ 6A, 85 ~ 185/185 ~ 265VAC PSU. Source: Digikey.

simulate this circuit – Schematic created using CircuitLab

Figure 2. (a) and (b) 240 V configuration. (c) and (d) 120 V configuration.

The peak voltage of the AC mains is $$\ \sqrt 2 V_{RMS} \$$. 240 V AC peaks at 340 V and 120 V AC peaks at 170 V.

• With the voltage selector switch open as shown in Figure 1a the circuit becomes a simple bridge rectifier as shown in !b. At 240 V AC there will be 340 V DC (approx.) between DC+ and DC-.
• With the voltage selector at 110 V the circuit becomes two simple half-wave rectifiers with 170 V DC across each capacitor resulting in the same 340 V DC between DC+ and DC-.

The linked schematic is worth studying. Note in particular their use of three different ground symbols.

simulate this circuit

Figure 3. Earth / ground symbols.

• They've used (a) as the chassis ground.
• They've used (c) as DC GND.
• My guess is that they only had one earth/ground symbol left so they used it, the earth symbol, to indicate the internal ground for the mains parts of the circuit. It is not connected to Earth. I would have tried to avoid using it in that way.

For more on ground symbols you can see what I've written on Ground, earth and chassis explained.